The present invention relates to an optical pickup capable of compatibly adopting disks of different thickness, and more particularly, to an optical pickup by which optical efficiency is improved by the adoption of at least two light sources.
A digital video disk player (hereinafter, called DVDP) should be capable of recording and reproducing information of not only a digital video disk (DVD) but also a compact disk (CD).
The standardized thickness of the DVD is 0.6 mm which is thinner than the standard thickness of the CD or CD-ROM of 1.2 mm. Since the standard thicknesses of the DVD and CD are different from each other, spherical aberrations are generated due to a difference in thickness when an optical pickup for a DVD reproduces information recorded on a CD, which makes information reproduction difficult.
To account for such problem, a conventional optical pickup which can read both a CD and a DVD has been devised, as shown in FIG. 1. The optical pickup shown in FIG. 1 includes an optical source 11 for producing and emitting laser light, an objective lens 19 for focusing incident light so that an optical spot is formed on the recording surface of an optical disk 30, a beam splitter 15 for changing the travel path of the incident light, and a hologram element 17 and an optical detector 23. The beam splitter 15 is positioned on a light path between the optical source 11 and the objective lens 19, and reflects most of the light received from the optical source 11 and passes most of the light reflected by the optical disk 30 toward the optical detector 23. The hologram element 17 is situated on a light path between the objective lens 19 and the beam splitter 15, and segregates light received from the optical source 11 into 0-order light 3 and +1-order light 5. The 0-order light 3 continues in the same direction as the incident light for the most part, and the +1-order light 5 diverges after passing through the hologram element 17. Thus, the 0-order and +1-order light 3 and 5 pass through different portions of the objective lens 19 and are focused at different positions. Here, the 0-order light 3 is used to record and reproduce information on the DVD, and the +1-order light 5 is used to record and reproduce information on the CD.
The optical detector 23 is comprised of a plurality of plates (not shown) each receiving light independently. Signals detected by the optical detector 23 are summed and/or subtracted, and the results are amplified to be outputted as an information signal (Radio Frequency), a tracking error signal and a focus error signal.
Also, the optical pickup further includes a grating 13 installed between the optical source 11 and the beam splitter 15 for diffracting the incident beam into 0-order diffracted beam and +1-order diffracted beam, and a collimating lens 16 installed between the beam splitter 15 and the hologram element 17 for changing emanated incident light into parallel light. Also, an astigmatism lens 21 installed between the beam splitter 15 and the optical detector 23 has a semicylinder shape in general. When the distance between the objective lens 19 and the optical disk 30 corresponds to an on-focus state, the astigmatism lens 21 allows a circular light to be formed on the optical detector 23, otherwise, it allows an elliptical light to be formed thereon.
The conventional optical pickup having such a configuration selectively employs the 0-order and +1-order light 3 and 5 diffracted in the hologram 17, so that the optical efficiency of a light spot formed on the optical disk 30 is degraded to less than 50%. Furthermore, light which is reflected by the optical disk 30 and proceeds toward the optical detector 23 via the hologram 17 has an efficiency of not more than 25%.